Determines whether the cells on the left or right side of the grid and ladder can be clicked to back or lay. The blue/pink colour also helps to remind you which setting you have chosen. Click blue to back, pink to lay. The setting chosen in each window only affects that Window.
On the Exchange Grid
The setting chosen only affects the Exchange Grid, it will not affect the polarity in any ladders.
On a Ladder
The setting chosen only affects that Ladder/Window, it will not affect the polarity in any other ladders. Each ladder has its own polarity setting.